For long-distance and large-capacity transmission through optical fiber, high-density signal multiplexing needs to be achieved and fiber non-linear optical effects need to be overcome.
Transmission capacity per optical fiber can be increased by performing high-density wavelength multiplexing in which different information items are placed on multiple optical carriers or optical subcarriers (subcarriers). Here, each of the multiplexed optical carriers or optical subcarriers will be referred to as a channel. The transmission capacity can also be increased by using multilevel modulation.
On off keying (OOK) that transmits one bit per symbol by assigning binary signals to the presence or absence of light has been conventionally used as a modulation method. The transmission capacity can be increased by increasing signal points to increase the number of bits transmitted per symbol like quaternary phase-shift keying (QPSK) or 16 quadrature amplitude modulation (QAM). In QPSK and 16 QAM, an optical transmitter assigns signals to an in-phase axis (I-axis) and a quadrature-phase axis (Q-axis).
A method of using polarization multiplexing to double the number of bits transmitted per symbol is also known. In polarization multiplexing, signals can be independently assigned to two orthogonal polarization components (vertically polarized wave and horizontally polarized wave).
In demodulation of an OOK signal, direct detection in which the presence or absence of an optical signal is detected and identified on a receiving side has been used. In demodulation of a differential binary phase-shift keying (DBPSK) signal, a differential QPSK (DQPSK) signal, and the like, (direct) delay detection that causes an optical signal to be delayed and interfere and then performs direct detection has been used. A digital coherent method has been often used for polarization-multiplexed signals (see, for example, Non Patent Literatures 1 and 2); the digital coherent method performs coherent detection at a receiving end to obtain an electrical signal and compensates the obtained electrical signal by digital signal processing; the coherent detection performs detection by causing a local oscillation light source and a received signal to be mixed and interfere with each other.
On the other hand, in long-distance optical transmission, depending on the bit rate, modulation method, and detection method, a predetermined optical signal power to noise power ratio is required to ensure signal quality at a receiving end, and thus a signal needs to be transmitted at a high optical power. In this case, waveform distortion due to non-linear optical effects occurring in optical fiber deteriorates signal quality (see, for example, Patent Literature 1).
The non-linear optical effects can be roughly separated into effects occurring in a channel and effects occurring between channels.
The non-linear optical effects occurring in a channel include self-phase modulation (SPM). As narrower definitions, the SPM is divided into intra-channel self-phase modulation (ISPM), intra-channel cross-phase modulation (IXPM), intra-channel four-wave mixing (IFWM), and the like.
The non-linear optical effects occurring between channels include cross-phase modulation (XPM), four-wave mixing (FWM), cross polarization modulation (XPolM), and the like.
Each of them significantly occurs when the optical power density of a signal is high and when the transmission distance is long. For the non-linear optical effects occurring between channels, the quality deteriorates significantly when the local wavelength dispersion in a transmission line is small, when a wavelength interval between wavelength-multiplexed channels is narrow, and when the polarization states of the optical signals of the respective channels are correlated in a transmission line for a long time and interaction continues.
In a polarization-multiplexed signal, the polarization state varies with the optical phase difference between the vertically polarized wave and the horizontally polarized wave. Thus, the relationship between data carried on the vertically polarized wave and data carried on the horizontally polarized wave affects the polarization state of the signal. Many of the data patterns depend on a user signal (client signal), so they are independent between the multiple channels. Thus, the relationship between data carried on the vertically polarized wave and data carried on the horizontally polarized wave is random between the channels, and naturally the polarization state of the signal is also random between the multiple channels.
FIG. 1 is an example illustrating a state in which correlation between the polarization states of three wavelengths λ1, λ2, and λ3 is low, and is an example in which of the vertical polarization (Y-Pol) and horizontal polarization (X-Pol), the polarization states of the three wavelengths are random for each symbol. In this case, the occurrence of the non-linear optical effects occurring between the channels is reduced.
However, in general, there are areas, such as frame overheads, in which data patterns are the same or non-independent between the multiple channels. FIG. 2 is a diagram illustrating an example of an optical transport unit level k(v) (OTUk(v)) frame format. An OTUk frame is composed of an overhead area, a payload area, and a forward error correction (FEC) parity area. A client signal is inserted in the payload area, and redundant bits for error correction are inserted in the FEC parity area. Monitoring information for a transmission line or information for frame synchronization are inserted in the overhead area. The frame format may be an OTUkV in which the proportion of the FEC parity area is non-standard. The overhead (OH) area may include an area having a parameter affecting a transmission line or a specific fixed pattern. However, if the areas in which the data patterns are non-independent include fixed patterns, the polarization states of the signals cannot be randomized between the multiple channels, and the influence of non-linear optical effects occurring between the channels may appear.
The times when the data patterns, such as the frame overheads, are non-independent generally differ between the multiple channels. In particular, because of difference between reference clocks of the multiple channels, the times when they are non-independent are out of synchronization with high probability. However, when frames are repeatedly transmitted, the times when the data patterns, such as the frame overheads, are non-independent can coincide between the multiple channels with low probability. At this time, the polarization states are correlated between the multiple channels, and unacceptable errors may occur at a receiver.
FIG. 3 is an example illustrating a state in which correlation between the polarization states of the three wavelengths λ1, λ2, and λ3 is high, and is an example in which of the vertical polarization (Y-Pol) and horizontal polarization (X-Pol), signals are concentrated only on the Y-Pol in the three wavelengths. In this case, the non-linear optical effects occurring between the channels occur significantly.
To prevent a situation where the polarization states of the optical signals of the respective channels are correlated in a transmission line for a long time, a method of randomizing polarization of a wavelength-multiplexed signal by using an optical component (polarization scrambler) to reduce non-linear optical effects occurring between channels has been proposed (see, for example, Patent Literature 2).
To reduce non-linear optical effects in a channel, a method of switching polarization of a signal alternately between orthogonal polarizations every symbol to reduce the optical power density of the signal in each polarization has been proposed (see, for example, Patent Literature 2). For a signal based on polarization multiplexing, a method of halving a pulse width and switching alternately between orthogonal polarizations every ½ symbol has also been proposed (see, for example, Patent Literature 3).
As a technique for compensating waveform distortion due to non-linear optical effects, a digital back propagation method of reproducing a signal at a transmitting end by simulating backward propagation in a fiber by digital signal processing is known (see, for example, Non Patent Literature 3). An optical phase conjugation method of canceling phase distortion at a receiving end by inverting the phase of light at a middle of a transmission line is also known (see, for example, Non Patent Literature 4).